MEXICO CITY — As common as shootings have become in Mexico, three deadly attacks Thursday stunned this nation for their brazenness: Four people leaving work at a border maquiladora were shot dead, seven young men were gunned down in a gritty neighborhood of Mexico City, and gunmen killed nine police officers in the western state of Jalisco.

The shootings follow several mass killings in less than a week. All have occurred in different parts of the country and appear unrelated.

Gunmen opened fire on three company buses in Caseta, a town near Ciudad Juarez along the U.S. border, killing a man and three women and injuring 15 others, local media reported.

"We have not seen an attack of this nature," Adrian Sanchez, a spokesman for the Ciudad Juarez police department, was quoted by the El Paso Times as saying. "We will have to wait to see what the investigation uncovers as to who did it. We can only give our condolences to the families of the people who lost their lives."

The gunmen reportedly boarded the buses and started shooting indiscriminately.

The workers were leaving Eagle Ottawa, a company based in Auburn Hills, Mich., that makes leather for vehicle interiors. In a statement the company said: "We are aware of this incident and we'll be responding from El Paso this evening, once we are able to better understand what happened."

A string of small towns in the border have been under siege from drug gangs trying to control trafficking routes. Mayors and police chiefs have been killed in the area, and even churches have been attacked.

Ciudad Juarez, across the border from El Paso, Texas, has become one of the world's deadliest cities since a turf war erupted nearly three years ago between the Juarez and the Sinaloa cartels. More than 6,500 people have been killed in the city since.

'Massacres have arrived in the Federal District'
In Mexico City, the men in their 20s and 30s were hanging out together on a street when the gunmen arrived. Mexico City Attorney General Miguel Angel Mancera said in an interview with the Televisa network that angry words were exchanged, and the gunmen opened fire.

The victims were said to be members of a drug gang that had been battling with another group in Tepito, a working-class neighborhood just north of Mexico City's colonial center.

The attack fueled fears of cartel-style violence reaching Mexico City, also known as the Federal District since it is the nation's capital.

"Massacres have arrived in the Federal District," declared El Universal newspaper, counting the shooting as the latest in a string of massacres that have occurred across the country over the past week.

Jalisco authorities complained that Michoacan officials had not joined in the search for the attackers.

And in Zapopan, a suburb of the Jalisco state capital of Guadalajara, two 2-year-old boys and three other people were wounded by grenade fragments during an attack on a private home.

The Zapopan municipal police department said the toddlers, a 17-year-old girl and two adults were all were bystanders on a street outside the home, and said none of the wounds appeared life threatening.

15 killed at car wash
The shootings occurred a day after gunmen killed 15 people at a car wash in Tepic, a city in the Pacific coast state of Nayarit.

Over the weekend, gunmen massacred 14 young people at a birthday party in the border city of Ciudad Juarez, and 13 recovering addicts were killed in an attack on a drug rehab center in Tijuana.

The first three shootings occurred in disputed drug trafficking territory where massacres, beheadings and shootouts occur frequently.

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While crime is a major problem in Mexico City, cartel-style violence has been less common.

Still, shootings between cartel gunmen and security forces have occasionally erupted during operations to arrest kingpins in the Mexico City area, one of the world's largest metropolises when an estimated 20 million people.

The most recent was the Aug. 20 capture of U.S.-born Edgar "La Barbie" Valdez Villarreal, a suspected drug lord who was allegedly fighting for control of Morelos, a state that borders the capital. While the arrest of "La Barbie" on the outskirts of Mexico City was peaceful, a shooting on a highway of the city that day killed one of his suspected accomplices.

Disputes between "La Barbie" and his rivals have also been blamed for several bodies found dumped in some neighborhoods of the capital this year.